High-temperature electric furnace



April 15, 1941. D. HANAWALT ET AL 3 7 HIGH-TEMPERATURE ELECTRIC FURNACEFiled July 28, 1939 Patenta& Apr, 35 3.941

HIGH-TEDMEMT URE ELECTRIC FURNACE 'Joseph lD; Hanawalt, (Charles E.Nelson, and John S. Peake,.Midlanl, Micic., aesignors to The DowChemical Compare Midlandl, mich., a co-poration ol Michigan ApplicationJuly 28, 3939, entel Ne.. %87,134

a claim& WL lui- 3313 This invention relates to an improved constructiontor electric furnaces operated at temperatures above 1000 C. Moreparticularly it relates to .the thermal insulation of such urnaces.

In various metallurgical processes, such as the liberaton andvaporization of magnesium and the alkaline earth metals from their ores,it is customary to operate in a graphite vessel at temperatures well inexcess of 1000 C., say at 1300- 1800 C., and preferably under vacuum orin the presence of an inert gas. Since it is a virtual impossibility toconstruct a completely vacuumtight vessel which will withstand suchtemperatures, it has been found necessary to enclose the heated vesselwithin a larger container or chamber which is at a much lowertemperature and hence can be made vacuum-tight, and to interpose a largebody of heat insulating material between the heated metallurgical vesseland the surrounding cool walls of the vacuum chamber.

OI the various possible heat-insulating materials which may be employedi'or this purpose, finely-divided carbon, e. g. lampblack or graphitedust, would seem highly preferable because it does not attack a graphitemetallurgical vessel even at temperaturesewell above 1000 C., andbecause at these temperatures it still has an outstandingly low thermaloonductvity. Unfortunately, however, finely-divided carbon, afterexposure to air, always contains large amounts of adsorbed gases,especially water, which, upon heating of the carbon, are eventuallyevolved or outgassed. Thus, when ordinary lampblack is used as the soleheat insulator in an electric vacuum furnace of the type described, theinsulation temperatures 'are such that water vapor and other gases areslowly desorbed and evolved within the vacuum chamber, graduallyattacking the metallurgical vessel and other graphite parts, andrequiring continuous removal. Thus, in a vacuum electric urnace ofmoderate commercial size, after each shut-down for cleaning or repairsduring which time the lampblack insulation is exposed to the air, thisoutgassing sometimes continues for a considerable period after operationof the fumace is resumed, during which time anumber of the graphiteparts may become so badly attacked as to decrease greatly the life of*the iurnace before another shut-down is necessai-y. In addition, forcertain metallurgical processes which must be operated at very lowpressure, this evolution of gas may be sufiicient to make it difiicultto attain the Operating pressure. For this reason the use offinely-divided carbonas the sole heat insulator for vacuum furrraces has"ri-een. considered impractical, and resort has seen had to othermaterials whichare poorer heat insulators and are often seriouslyafiected by contact with 'the inner graphite vessel at the hightemperatures.

We have now' "found, however, that it is not only possible but highlydesirable to use lamphlacl; or other nely-dvded carbon as the principalthermal insulator in electric Vacuum furnaces, provided the umace be soconstructed that during operation substantially all the mass oflampblack is maintained above its desorption temperature, i. e. abovethe temperature at which the lampblacis will retain adsorbed gases ormore than a short period, usually above about 500 C. 'When thistemperature condition is met, outgassing of the nely-divided carhoninsulation is completed within a relatively short period after operationof the furnace is begun, the gases being entirely removed before thereis time for any significant attack on the iurnace parts to occur. It isthus possible to utilize the excellent thermal properties and Chemicalinertness of the finely-divided carbon insulation without encounteringthe outgassing diiculties of prior art prec tice. Maintenance costs arelowered and the sizes of the Iurnaces may he reduced below that ofi'urnaces using poorer insulating material. Any form of finely-dividedcarbon, i. e. carbon having a particle size be1ow 0.1 mm., may heemployed.

A variety of means may be used in an eiectric y furnace according to theinvention to maintain the flnely-divided carbon insulation above itsdesorption temperature; for instance, heaters may be placed in the.carbon. However, the simplest construction in practice is to dispose thenely divided carbon insulation immediately around the graphite furnacechamber where it serves as the primar-y insulation, being exposed to thehighest temperatures and most severe conditions, and then to surroundthis carbon with another or secondary heat-insulating material. Thissecondary insulation is employed in such thickness that the thermalgradient therein is sufiicient' to insure that the finely-divided carboninsulation is at a temperature at least as high as the desorptlontemperature of the carbon. The secondary insulation should not, however,be so thick that the temperature at the carbon insulation-secondaryinsulation interface is so high that the Chemical interaction betweenthe two materials might occur. In this Construction the secondarythermal insulation serves chiefiy as a lining or blanket to keep thecarbon insulation g aas-lar e at the desired temperature. It is itselfnot ex- `posed to the extreme temperatures of the furnace interior andhence may be made of any reiractory material not subject to outgassing,e. g. rebrick, magnesite brick, etc. or combinations thereof.

One form of apparatus utilizing the principle of the invention isillustrated in the accompanying drawing which shows an electric-vacuumfurnace in diagrammatic vertical section. such a furnace finds use, :forexample in metallurgcal processes in which a solid ore charge isreduced, liberating metal in vapor form, and leaving a solid resiclualslag.

Referring to the drawing, the entire furnace .structure is 'shown housedin an air-tight shell i made of steel or other suitable material andopen at the top through a manhole 2 having an air tight cover 3 With avalved outlet d. vounted within the chamb-er i on gran-hite piers 5 is agraphite metallurgical vessel ti having a sloping hearth ll and a slagpit and provided with a charge inlet a vapor outlet w, and a slag outletll. solid material entel-s the inlet 9 through a charging lock [12, andgas may enter through a valvecl inlet 03. Slag leaves the iurnace outletll through a discharge lock l l. The inetallurgical charnber G is heatedby electric resistor bars iii which receive current through suitablegast'ght leads not illustrated. The principal furhace insulation is abed of lampblack i@ in contact with and entirely surrounding the vesselBetween the lampblack and the walls or the chamber i is a liningconsisting of layers of secondary insulation, i. e. magnesite brickIl'l, chrome brick iS, and ordinary firebrick 09. 'The lampblaclg iscovered on top by a mat of rock wool which also serves as seconclaryheat insulation.

in operation, current'is passed through the resistors 15 so that thetemperature in the graphite vessel 6 is well above 1000 C., say at1300-i800 C., the exact temperature depending upon the particularmetallurgical process being carried out. A charge of ore is introduceclthrough the .inlet il, and falls onto the hearth ll, where under thetemperature prevailing it reacts to liberate metal vapor, which escapesthrough the outlet i@ to a condenser not shown; any slag formed fallsinto the pit from which it may be withdrawn through the outlet M. "Whenit is desired to operate under reduced pres'sure, the v alve t! isclosed. and vacuum is appliecl through the outlet lt When Operating inan inert atmosphere, the inert gas may be introduced through the inlet03 and allowed to escape through theoutlet W. in either case, thefurnace is being operated in the absence In working under vacuum, whenthe iurnace is first started up, the lampblacl i@ evolves adsorbedgases. These gases may be withdrawn from the chanber l by applying avacuurn to the outlet l, which is then closed when outgassing iscomplete. In an alternative construction, the outlet 6 may be omitted,the evolved gas es seening through the graphite walls of the vessel Gand being withdrawn through the outlet lil. It will be appreciated thatbecause of the presence of the secondary insulation il, 063, se, and ao,the lampblack (i is maintained well above its desorption temperature,and the initial outgassing just rnentionecl` is complete within acompare,- tively short period. The structure illustrated thus utilizestothe iullest extent the ideal thermal properties and Chemical inertnessof lamplolack,

and yet avoids the outgassing difficulties of previous iurneces.

'The relative thiclrnesses of the finely-divided carbon insulation andthe secondary insulation necessary to insure that the ormer remainsabove its desorption temperature, i. e. above about 500 C., when thevessel G-is at temperatures above lll0 C., may be oomputeol by familiarthermal principles from the known heat conductivities of the variousrefractory materials used. In the most economical Construction, thesecondary in` sulation is just thick enough to keep the outer portion ofthe lampblach just above the desorio tion temperature, hut greaterthicknesses, such that the lampblacl; is at much higher temperaturesthroughout, may he employed.

lt is to he understood that the invention is not limited to the specificstructure shown but is applicahle to any high temperature furnaceutilizing electric heating means and operating at above i C. in theabsence of air, i. e. under reduced pressure or with an inertatmosphere.

Vie claim:

i. An electrical iurnaoe for operation at temperatures above NOW C.comprising: an air-tight shell; a graphite furnace chamber supportedwithin the shell; electric heating means within the chamber; a bed oflampblaci: immediately surrounding the chamber and acting as a priinarytherinal insul-ation between the chamber and the shell; and means iormaintaining the said lampblaci substantially all at a temperature aboveeow 0.; and .means for removing gases ironi within the shell.

2. .in an electric urnace, the oombination of: an eir-tight shell; agraphite urnace chainber supported within the shell; electric heatingmeans within the chamber; a bed of fnely-clivided carhon immediatelysurrounding the chamber and acting as primary thermal insulation betweenthe chainloer and the shell; and a. lining ofheat reiractory secondaryinsulating material between the finely divided carhon and the shell,said material being of such thickness that when the furnace is operatingat temperatures above 1000 C. the therrnal gredient in such reiractoryis sufrlcient to maintain the finely-divcled carbon substantially all ata temperature above its desorption temperature, and means :for removinggases from within the shell.

3. in an electric furnace, the combination of: an air-tight shell; agraphite iurnace chamber supported within the shell; electric heatingmeans within the chamber; a bed of nely-divlcled carbon immediatelysurrounding the ohamber and acting as primary thermal insulati'onbetween the chamber and the shell; and a lining of heat refractoryseconclary insulating material between the nely-divided carbon and theshell, said material being of such thickness that when the iurnace isOperating at temperatures above iooo C. the therrnal gradient in suchrefractory is sufcient to maintain the firely-divioled carhonsuhstantially at a temperature above 0 C.: and means for removing gasesfroni within the shell.

4. in an electric furnace, the combinaton of: an air-tight shell; agraphite furnace chamber supported within the shell; electricheatingmeans within the chamber; e charge inlet extending :from outside theshell into the said. chamber; a bed of lampblacl ;immediatelysurrounding the chamber and actng as primary insulation between thechamber and the shell; and a lining o refractory secondary heatinsulating material between the lampblack and the shell, said materialbeing of such thickness that when the furnace is Operating attemperatures above 1000 C. the thermal gradient in said refractory issuch as to maintain the lampblack substantially all at a temperatureabove 500 C.; and means for removing gases from within the shell.

5. In an electric vacuum fumace, the combination of: an air-tight shell;a graphite furnace chamber supported within the shell; electric heatingmeans within the chamber; a vapor outlet leading from said chamber tooutside the shell; a bed of finely-divided carbon immediatelysurrounding the chamber and acting as primary thermal insulation betweenthe chamber and the shell; and a lning of refractory secondaryheatinsulating material between thecarbon and the shell, said materialbeing of such thickness that when the furnace is Operating at atemperature above 1000 C. the therma gradient in said refractory is suchas to maintain the carbon substantially all at a temperature, above 500C.

6. In an electric vacuum furnace, the combination of: an air-tightshell; a graphite furnace chamber supported within the shell; electricheating means within the chamber; a charge inlet extending from outsidethe shell into the said chamber; a vacuum charge lock in the said chargeinlet; a slag outlet leading from said chamber to outside the shell; avacuum discharge lock in the said outlet; a vapor outlet leading fromsaid chamber to outside the shell; a bed of lampblack immediatelysurrounding the chamber and, acting as primary insulation between thechamber and the shell; and a lining of. refractory secondary heatinsulating material between the lampblack and the shell, said materialbeing of such thickness that when the urnace is Operating attemperatures above 1000 C. the thermal gradient in said refractory issuch as to maintain the lampblack substantially all at a temperatureabove 500 C.

7. In an electric vacuum furnace, the combi- I nation of: an air-tightshell; a valved gas outlet in said shell; a graphite furnace chambermounted on refractory piers within the shell; electric resistanceheating elements within the chamber; a charge inlet extending fromoutside the shell into the .chamber; a vacuum charge lock and a valvedgas inletin the saidtcharge inlet; a slag outlet leading from saidchamber to outside the shell; a vacuum discharge lock in said outlet; avapor outlet leading from said chamber to outside the shell; a bed oflampblack immediately surrounding the chamber and acting as primaryinsulation between the chamber and the shell; and a lining of refractorysecondary heat insulating material between the lampblack and the shell,said material being of such thickness that when the fumace is Operatingat temperatures above 1000 C. the thermal gradient in said refractory issuch as to maintain the lampblack substantially all at a. temperatureabove 500 C.

8. In an electric furnace, the combination of: -an air-tight shell; agraphite fu'nace chamber supported within the shell; electric heatingmeans within the chamber; a bed of finely-divided carbon immediatelysurrounding the chamber and acting as a primary thermal lnsulationbetween the chamber and the shell; and a linin! of secondary thermalinsulation between said bed of carbon and the shell consisting ofsuccessive layers of magnesite brick, chrome brick, and firebrick, thesaid brick layers being of suflicient thickness that when the furnace isOperating at temperatures above 1000 C. the thermal gradient in thebrick is such as to maintain the primary insulation all at a temperatureabove 500 C., and means for removing gases from within the shell.

JOSEPH D. HANAWALT. CHARLES E. NELSON. JOHN S. PEAKE.

